This invention relates to an improved method for manufacturing silica waveguides with minimal absorption.
The manufacture of optical devices which employ silica waveguides, such as optical Multiplexers (Mux) and Demultiplexers (Dmux), entails depositing silica films onto a silicon wafer. The silica films are ideally of optical quality, characterised in that they are transparent in the 1.30 xcexcm bi-directional narrow optical band and/or in the 1.55 xcexcm video signal optical band. Such optical quality silica films are extremely difficult to produce in reality because hydrogen and nitrogen atoms are typically present in the films. These impurities in the silica films result in excessive optical absorption in the 1.30 and 1.55 xcexcm optical bands.
Fourier Transform Infrared (FTIR) spectroscopy can be used to monitor the quality of optical silica films. The FTIR spectra of optical quality silica films, containing no undesirable optical absorption peaks, are characterised by the presence of only four fundamental optical absorption peaks: (1) an intense and small Full Width at Half Maximum (FWHM) Sixe2x80x94Oxe2x80x94Si xe2x80x9crocking modexe2x80x9d absorption peak ranging between 410 and 510 cmxe2x88x921, centred at 460 cmxe2x88x921 (21.739 xcexcm); (2) a small FWHM Sixe2x80x94Oxe2x80x94Si xe2x80x9cbending modexe2x80x9d absorption peak ranging between 740 and 880 cmxe2x88x921, centred at 810 cmxe2x88x921 (12.346 xcexcm); (3) an intense and small Full Width at Half Maximum (FWHM) Sixe2x80x94Oxe2x80x94Si xe2x80x9cin-phase-stretching modexe2x80x9d absorption peak ranging between 1000 and 1160 cmxe2x88x921, centred at 1080 cmxe2x88x921 (9.256 xcexcm) indicating stoichiometric silica films with the optimum Sixe2x80x94Oxe2x80x94Si bond angle of 144xc2x0 and the optimum density; and (4) an almost eliminated Sixe2x80x94Oxe2x80x94Si xe2x80x9cout-of-phase-stretching modexe2x80x9d absorption peak ranging between 1080 and 1280 cmxe2x88x921, centred at 1180 cmxe2x88x921 (8.475 xcexcm), as compared to the Sixe2x80x94Oxe2x80x94Si in-phase-stretching mode absorption peak.
The position in the infrared spectrum of these four fundamental (first mode) infrared absorption peaks, respectively centered at 21.739 xcexcm, 12.346 xcexcm, 9.256 xcexcm, and 8.475 xcexcm, is far away from the infrared bands of interest at 1.30 and 1.55 xcexcm. However, residual absorption of optical quality silica is never completely eliminated because the higher harmonics of these four residual optical absorption peaks do cause small residual optical absorption peaks in the 1.30 and 1.55 xcexcm optical band. The very high harmonics (i.e. very little absorption effect) falling within this range are: the sixth (1.302 to 1.543 xcexcm) and seventh (1.116 to 1.323 xcexcm) harmonics of the Sixe2x80x94Oxe2x80x94Si xe2x80x9cout-of-phase-stretching modexe2x80x9d infrared absorption peak; the sixth (1.437 to 1.667 xcexcm) and seventh (1.232 to 1.429 xcexcm) harmonics of the Sixe2x80x94Oxe2x80x94Si xe2x80x9cin-phase-stretching modexe2x80x9d infrared absorption peak; the eighth (1.420 to 1.689 xcexcm) and ninth (1.263 to 1.502 xcexcm) harmonics of the Sixe2x80x94Oxe2x80x94Si xe2x80x9cbending modexe2x80x9d infrared absorption peak; and the thirteenth (1.508 to 1.876 xcexcm) and fourteenth (1.401 to 1.742 xcexcm) and fifteenth (1.307 to 1.626 xcexcm) harmonics of the Sixe2x80x94Oxe2x80x94Si xe2x80x9crocking modexe2x80x9d infrared absorption peak.
The FTIR spectra of optical quality silica films are also characterised by a net separation between the Sixe2x80x94Oxe2x80x94Si xe2x80x9cin-phase-stretching modexe2x80x9d absorption peak (1080 cmxe2x88x921) and the Sixe2x80x94Oxe2x80x94Si xe2x80x9cbending modexe2x80x9d absorption peak (810 cmxe2x88x921) with a deep valley between 850 and 1000 cmxe2x88x921.
Silica films may be deposited onto a silicon wafer using a silane (SiH4) and nitrous oxide (N2O) gas mixture at a low temperature according to the following reaction:
SiH4(g)+2N2O(g)xe2x86x92SiO2+2N2(g)+2H2(g)
Theoretically, it is possible to achieve optical quality silica films from this reaction. However, in reality, numerous side reactions occur, forming a mixture of undesirable Sixe2x80x94Oxxe2x80x94Hyxe2x80x94Nz compounds. For example, FIG. 1 presents the various potential Sixe2x80x94Oxxe2x80x94Hyxe2x80x94Nz compounds that may result from the combination of silane (SiH4) and nitrous oxide (N2O) gases. It shows 35 products that could be found in silica films deposited from a silane (SiH4) and nitrous oxide (N2O) gas mixture. N2, O2, HNO, NH3, H2O, and H2 gaseous by-products are eliminated from the surface or from the micro-pores of the silica films during these chemical reactions. As a result of the production of these side-products, the incorporation of oxygen atoms, a key factor to achieve optical quality silica, competes with the incorporation of nitrogen and hydrogen atoms in the silica films. Thus, the silica films as deposited on the silicon wafer are not optical quality silica films, due to the absorption by the undesirable Sixe2x80x94Oxxe2x80x94Hyxe2x80x94Nz compounds formed.
To resolve this problem caused by Sixe2x80x94Oxxe2x80x94Hyxe2x80x94Nz impurities in the films, techniques have been used wherein the silica films are subject to a high temperature (typically, between 600 and 1350xc2x0 C.) thermal treatment under vacuum, argon (Ar), or a nitrogen atmosphere as a means for reducing the optical absorption of silica films in the 1.30 and 1.55 xcexcm optical regions. In general, the higher the temperature of this high temperature thermal treatment, the lower the optical absorption of the silica films. However, unlike fused silica optical fibres, that are heated at a temperature exceeding about 2000xc2x0 C. during the drawing process, the high temperature thermal treatment of the silica films on silicon wafers is performed at a temperature ranging from 600xc2x0 C. to a maximum temperature of about 1350xc2x0 C., close to the fusion point of the silicon wafer. The temperature is typically limited by the high compressive mechanical stress induced in the silica films from the difference of thermal expansion between the silica films and the underlying silicon wafer. This temperature limitation results in silica films with undesirable residual infrared oscillators and in their associated undesirable residual optical absorption peaks in the 1.30 and 1.55 xcexcm wavelength optical bands.
Thus, using a high temperature thermal treatment in the presence of nitrogen on the thirty-five products of silane and nitrous oxide given in FIG. 1, results in a maximum of only twelve of the thirty-five potential Sixe2x80x94Oxxe2x80x94Hyxe2x80x94Nz products being converted to SiO2. The same twelve compounds could also lead to the formation of SiO2 in an inert (Ar) atmosphere or under vacuum, since in none of these twelve chemical reactions is nitrogen incorporated.
Following a high temperature thermal treatment in a nitrogen atmosphere, the other twenty-three Sixe2x80x94Oxxe2x80x94Hyxe2x80x94Nz potential compounds can lead to the formation of: SiNH, SiN2, SiOH2, SiONH, and SiON2. Therefore, high temperature thermal treatments under nitrogen, argon, or in a vacuum are incapable of transforming twenty-three potential initial Sixe2x80x94Oxxe2x80x94Hyxe2x80x94Nz products formed from the reaction of silane and nitrous oxide into SiO2. Thus, the silica films that result from these high temperature thermal treatments under nitrogen, argon, or in a vacuum are composed not only of SiO2, but are solid mixtures of six possible compounds: SiO2, SiNH, SiN2, SiOH2, SiONH and SiON2. Gaseous by-products that result from the thermal decomposition of silica films are: nitrogen (N2), hydrogen (H2), and ammonia (NH3).
FIG. 3 lists some FTIR fundamental infrared absorption peaks and their corresponding higher harmonic peaks associated with SiO2, SiNH, SiN2, SiOH2, SiONH, and SiON2. The higher harmonics of the absorption peaks corresponding to these six residual potential compounds contribute to the optical absorption in the 1.30 and 1.55 xcexcm optical bands, as follows: the second vibration harmonics of the HOxe2x80x94H oscillators in trapped water vapour in the micro-pores of the silica films (3550 to 3750 cmxe2x88x921) increases the optical absorption near 1.333 to 1.408 xcexcm; the second vibration harmonics of the SiOxe2x80x94H oscillators in the silica films (3470 to 3550 cmxe2x88x921) increases the optical absorption near 1.408 to 1.441 xcexcm; the second vibration harmonics of the SiNxe2x80x94H oscillators in the silica films (3380 to 3460 cmxe2x88x921) increases the optical absorption near 1.445 to 1.479 xcexcm; the second vibration harmonics of the SiNxe2x80x94H oscillators in the silica films (3300 to 3460 cmxe2x88x921) increases the optical absorption near 1.445 to 1.515 xcexcm; the third vibration harmonics of the Sixe2x80x94H oscillators in the silica films (2210 to 2310 cmxe2x88x921) increases the optical absorption near 1.443 to 1.505 xcexcm; the fourth vibration harmonics of the Sixe2x95x90O oscillators in the silica films (1800 to 1950 cmxe2x88x921) increases the optical absorption near 1.282 to 1.389 xcexcm; and the fifth vibration harmonics of the Nxe2x95x90N oscillators in the silica films (1530 to 1580 cmxe2x88x921) increases the optical absorption near 1.266 to 1.307 xcexcm. The negative effects of these the oscillators on the optical properties of silica films are reported in the literature.
Thus, this high temperature thermal treatment of silica films under vacuum, argon, or nitrogen, does not provide a very efficient way to eliminate the excessive absorption at various wavelengths in the 1.30 and 1.55 xcexcm optical bands.
An object of the invention is to alleviate the afore-mentioned problems.
In one aspect, the invention provides a method of making a high optical quality silica film, comprising: subjecting a substrate to a gaseous mixture of silane and nitrous oxide to deposit said film on said substrate in accordance with the reaction
SiH4(g)+2N2O(g)xe2x86x92SiO2+2N2(g)+2H2(g)
and subsequently subjecting said deposited film to a reactive atmosphere containing hydrogen and oxygen atoms to chemically transform impurities resulting from the reaction into pure silica.
In another aspect the invention provides a method for reducing the optical absorbance of a silica film coated on a substrate, comprising: subjecting the silica film coated on the substrate to a temperature of about 600xc2x0 to about 1000xc2x0 C. under nitrogen, an inert atmosphere, or under vacuum; increasing the temperature to a maximum temperature of at most 1350xc2x0 C.; exposing the silica film to a reactive atmosphere comprising oxygen and hydrogen atoms by replacing the nitrogen, inert atmosphere or vacuum by the reactive atmosphere; removing the silica film from the reactive atmosphere by replacing the reactive atmosphere with nitrogen, an inert atmosphere, or vacuum; decreasing the temperature from the maximum temperature to about 1000xc2x0 to about 600xc2x0 C.; and recovering the silica film coated on the substrate.